Space debris from discarded upper stages, satellites not in use, and assorted pieces from staging, tank explosions, and impacts has been growing for the past 50 years. There are currently about 9,000 tracked debris objects in low Earth orbits per 450 operational satellites (i.e., 20 to 1 ratio). The number of untracked fragments in the centimeter range which can be lethal to operational satellites is simply staggering, on the order of 500,000.
It is understood now that after years of debris accumulation, the debris cloud in low Earth orbits has crossed critical density thresholds over a wide range of altitudes, and entered into the phase of accelerated debris creation in collisions that gradually become more and more frequent. In this deteriorating environment, catastrophic collisions are becoming a reality. On Feb. 10, 2009, derelict Cosmos 2251 collided with operational Iridium 33 at 11.6 km/s. In less than a millisecond, the two satellites disintegrated, producing nearly 2,000 tracked debris object fragments and on the order of 100,000 untracked debris object fragments in the centimeter range. Theory predicts that we may witness another catastrophic collision in this decade. With each collision, the produced debris dramatically increase the risk to active satellites and the need for avoidance maneuvering.
It is also understood that large debris objects, such as old upper stages, should be removed first, because they are typically the primary source of many thousands of small fragments generated in collisions between large objects. These small fragments are like bullets, whizzing all around at orbital speeds and capable of disabling operational satellites upon impact. They are too small to track and avoid, but too heavy to shield against. Unless the source of new fragments is removed, the near-Earth orbits may be rendered unusable.
The task of removing large space debris is enormous. There are over 2,000 large debris objects totaling 2,000 tons scattered throughout low Earth orbits. Many, or preferably, substantially all of these objects should be removed to substantially reduce the risk of debris generation in collisions. So far, debris cleanup has not been attempted, because no practical solutions have been developed. It has been estimated that sending rockets to remove large debris would be very expensive: the cost per kilogram of debris removed would exceed a typical launch cost per kilogram. This cost is prohibitive.
Another serious problem is the method of disposal of debris. One way would be to bring the debris objects to low orbits and let them reenter the atmosphere and burn in the atmosphere. However, large objects do not burn completely, and there are serious concerns about liability of atmospheric reentry of many large objects.
Another concern is the wasted value of these objects. It was very expensive to launch them to orbit. It would be advantageous to make some use of it, and there is a need. On the one hand, we have thousands of tons of “scrap material” circling the Earth, but on the other hand, we need to launch thousands of tons of parts and equipment to build space hotels and habitats, fuel depots, and space manufacturing and servicing facilities.
In 2010, NASA formulated new Grand Challenges in Space Technology. One of them is the Challenge of Space Debris Hazard Mitigation. It is acknowledged that “mitigation is difficult and requires solutions that are practical, yet technically and economically feasible.” But, there is also the Challenge of Space Way Station, which seeks to “develop pre-stationed and in-situ resource capabilities, along with in-space manufacturing, storage and repair to replenish the resources for sustaining life and mobility in space.” It is acknowledged that the “current capabilities are insufficient to extract, refine, form stock, and transport in-situ materials for in-space manufacturing, servicing, fueling and repair. In-space system repair and maintenance is cost-prohibitive and difficult, consequently, many spacecraft are de-orbited at end-of life.”